Impact of a micromultileaf collimator on stereotactic radiotherapy of uveal melanoma

Comparison of stereotactic radiotherapy and protons for uveal melanoma patients

Background and purpose

Uveal melanoma (UM) is the most common primary ocular malignancy. We compared fractionated stereotactic radiotherapy (SRT) with proton therapy, including toxicity risks for UM patients.

Materials and methods

For a total of 66 UM patients from a single center, SRT dose distributions were compared to protons using the same planning CT. Fourteen dose-volume parameters were compared in 2-Gy equivalent dose per fraction (EQD2). Four toxicity profiles were evaluated: maculopathy, optic-neuropathy, visual acuity impairment (Profile I); neovascular glaucoma (Profile II); radiation-induced retinopathy (Profile III); and dry-eye syndrome (Profile IV). For Profile III, retina Mercator maps were generated to visualize the geographical location of dose differences.

Results

In 9/66 cases, (14 %) proton plans were superior for all dose-volume parameters. Higher T stages benefited more from protons in Profile I, especially tumors located within 3 mm or less from the optic nerve. In Profile II, only 9/66 cases resulted in a better proton plan. In Profile III, better retina volume sparing was always achievable with protons, with a larger gain for T3 tumors. In Profile IV, protons always reduced the risk of toxicity with a median RBE-weighted EQD2 reduction of 15.3 Gy.

Conclusions

This study reports the first side-by-side imaging-based planning comparison between protons and SRT for UM patients. Globally, while protons appear almost always better regarding the risk of optic-neuropathy, retinopathy and dry-eye syndrome, for other toxicity like neovascular glaucoma, a plan comparison is warranted. Choice would depend on the prioritization of risks.

Feasibility, Method and Early Outcome of Image-Guided Volumetric Modulated Arc Radiosurgery Followed by Resection for AJCC Stage IIA–IIIB High-Risk Large Intraocular Melanoma

Simple Summary

The aim of this trial was to define one optimal contemporary treatment procedure for large intraocular melanoma. Radiosurgery is a highly effective treatment in cancer. In this trial, all consecutive patients with large intraocular melanoma treated with multimodality treatment, comprising 4D image-guided volumetric modulated arc radiosurgery procedure followed by resection, were evaluated. In the short-term follow-up there was no clinical toxicity due to external beam radiation therapy, and no local tumor recurrence. In 98% of the cases, the eye bulb could be maintained with partial residual visual acuity in the mean follow-up of 18 months. The outcome estimates one optimal treatment procedure for high-risk, large intraocular melanoma, with excellent results in the first follow-up.

Abstract

The main objective of this prospective observational study was the characterization of the feasibility and early outcome of image-guided (IG) volumetric modulated arc (VMAT) radiosurgery (SRS) followed by resection for patients with large intraocular melanoma. Our study included consecutive patients with unfavorable-risk melanoma, enrolled in an ophthalmic oncology center. IG-VMAT-SRS was applied by high-resolution 4D image guidance and monitoring. Current stereotactic technique parameters were evaluated for comparison. Side effects and eye function, based on a 5-point CTC assessment score, were quantified. In patients with tumors located more than 0.7–1 mm apart from the optic nerve, partial to complete volume-sparing of the optic nerve head could be achieved. In 95.5% of this subgroup, the vitality of the optic nerve and vision could be preserved by the multimodality-treatment approach (mean follow-up: 18 months (7.5–36 months)). The advanced technology of stereotactic radiotherapy demonstrated the achievability of steep dose gradients around the high-dose volume, with 4D-IG-VMAT dose application. These results enforce IG-VMAT-SRS followed by resection as one of the major therapeutic options for patients with large intraocular melanoma. The combination of 4D-IG high-precision SRS and resection provides an effective treatment for large intraocular melanoma, with few side effects, and enables an eye bulb and even vision preserving modus operandi.

Dosimetric comparison and secondary malignancy risk estimation for linac-based and robotic stereotactic radiotherapy in uveal melanoma

It was aimed to investigate the dosimetric differences among linac-based and robotic stereotactic radiotherapy (SRT) techniques for the treatment of uveal melanoma and to evaluate secondary malignancy risks for these different SRT techniques. Ten patients who received robotic SRT with CyberKnife were retrospectively included in this study. A total dose of 54 Gy in three fractions was prescribed to the planning target volume (PTV). For each patient, non-coplanar micro-multileaf collimator based dynamic conformal arc (DCA), intensity-modulated radiotherapy (IMRT) and circular cone based DCA (cDCA) plans were generated. During the analysis dose-volume histogram (DVH) parameters, homogeneity index, new conformity index, the volume received more than or equal to 30% and 50% of the prescribed dose were compared. Additionally, secondary malignancy risk for each technique was estimated using the risk factors recommended by The International Commission on Radiological Protection. Robotic SRT plans provided a high degree of conformity within the PTV and better normal tissue sparing compared to linac-based treatment plans. However, dose distribution was more heterogeneous in robotic SRT plans than that in linac-based techniques. Estimated secondary malignancy risk was also found as 3.4%, 1.4%, 1.4% and 1.6% for robotic SRT and linac-based IMRT, DCA, cDCA plans, respectively. Treatment parameters of uveal melanoma patients planned with robotic SRT had superior conformity and organ-at-risk (OAR) sparing compared with those planned with the linac-based system. However, estimated secondary malignancy risk was almost two-times higher in robotic SRT than that in linac-based techniques.

Hypofractionated stereotactic photon radiotherapy of choroidal melanoma: 20-year experience

BACKGROUND

To evaluate the long-term results after hypofractionated stereotactic photon radiotherapy (SRT) in patients with choroidal melanoma treated between 1997 and 2016.

MATERIAL AND METHODS

A total of 335 patients (183 male and 152 female) with choroidal melanoma unsuitable for ruthenium-106 brachytherapy or local resection were treated with linear accelerator-based SRT at the Medical University of Vienna. All patients received five fractions with either 10, 12 or 14 Gy per fraction. A complete ophthalmic examination including visual acuity and measurement of the tumor base and height using standardized A- and B-scan ultrasonography was performed every 3 months in the first 2 years, every 6 months until 5 years and yearly thereafter. Early and late adverse side effects were assessed at every follow-up visit.

RESULTS

The median overall follow-up was 78.6 months (39.1 to 113.7 months). Local tumor control was 95.4% after 10 and 12 years, respectively. Fifty-four patients developed metastatic disease, and 31 died during the follow-up. Mean visual acuity decreased from 0.55 Snellen at baseline to 0.05 Snellen at the last individual follow-up. Ischemic retinopathy (192/335cases) and optic neuropathy (174/335cases) were the most common radiogenic side effects, followed by radiogenic cataract (n = 127), neovascular glaucoma (n = 71) and corneal epithelium defects (n = 49). Enucleation was performed in 54 patients mostly due to neovascular glaucoma (n = 41) or tumor recurrence (n = 10) during the study period. The eye retention rate was 79.7% after 10 and 12 years.

CONCLUSION

Hypofractionated stereotactic photon radiotherapy showed a high rate of local tumor control for choroidal melanoma and an acceptable rate of radiogenic side effects.

Early ultrasonographic tumor regression after linear accelerator stereotactic fractionated photon radiotherapy of choroidal melanoma as a predictor for metastatic spread

BACKGROUND AND PURPOSE

During extended follow-up (of up to 15 years), approximately fifty percent of patients with choroidal melanoma will develop metastatic disease and eventually die. Thus, continuing research on prognostic factors, early detection and treatment is necessary. Height regression rates both after plaque brachytherapy and proton beam irradiation have been shown to have prognostic value. The purpose of this study was to analyze the influence of early tumor regression rate after treatment of choroidal melanoma with LINAC stereotactic fractionated radiotherapy (SFRT) as an independent risk factor for metastasis.

MATERIAL AND METHODS

256 patients with choroidal melanoma treated with LINAC SFRT were included. Follow-up included standardized echography yielding apical height, smallest and largest basal linear diameter, tumor volume and mean reflectivity. The influence of baseline measurements and of a longitudinal, normalized area under the curve coefficient (NAC) of the latter marker on metastasis risk was assessed.

RESULTS

NAC for tumor thickness at months 3, 6, and 12 had a statistically significant (p < 0.001) non-linear effect on risk of metastasis. Additionally, ultrasonographic baseline tumor dimensions, but not internal reflectivity were found to be statistically significant risk factors for metastasis.

CONCLUSIONS

Our results demonstrate a non-linear influence of regression rate of choroidal melanoma as independent risk factor of metastatic disease after LINAC SFRT. These prove the clinical experience that, in comparison to rather slow regressions, very quick and very slow early tumor responses to LINAC SFRT are associated with a significantly higher metastasis risk.

Optimizing radiosurgery with photons for ocular melanoma

BACKGROUND AND PURPOSE

Photon radiotherapy has been established for the treatment of ocular melanoma (OM). Here we investigate the planning qualities of two different planning approaches, a combination of dynamic conformal arcs (DCA) complemented with multiple non-coplanar static intensity-modulated (IMRT) fields (DCA-IMRT), and volumetric modulated arc therapy (VMAT) in combination with automated planning (AP).

MATERIALS AND METHODS

Thirteen consecutive patients treated for ocular melanoma with curative intent on a Linac-based radiosurgery system were analyzed. Fractionated stereotactic radiosurgery (fSRS) was applied using 50 Gy in 5 fractions using the combination of DCA-IMRT. Plans were reviewed and the thirteen cases were compared to plans obtained with optimized automated VMAT based on a set of 28 distinct patients treated with DCA-IMRT who were selected to generate the AP model for the prediction of dose volume constraints.

RESULTS

Overall, plan quality of DCA-IMRT was superior to AP with VMAT. PTV coverage did not exceed 107% in any case treated with DCA-IMRT, compared to seven patients with VMAT. The median PTV covered by >95% was 98.3% (91.9%-99.7%) with DCA-IMRT, compared to 95.1% (91.5%-97.9%) (p < 0.01) with VMAT. The median mean dose delivered to the treated eye was 22.4 Gy (12.3 Gy-33.3 Gy) with DCA-IMRT compared to 27.2 Gy (15.5 Gy-33.7 Gy) (p < 0.01). Dose to the ipsilateral lacrimal gland and the ipsilateral optic nerve were comparable for DCA-IMRT and VMAT, however, the dose to the lens was lower with DCA-IMRT compared to VMAT.

CONCLUSIONS

The combination of multiple arcs complemented with multiple IMRT fields sets the gold standard for fSRS of ocular melanoma for photon therapy.

Feasibility study of a non-invasive eye fixation and monitoring device using a right-angle prism mirror for intensity-modulated radiotherapy for choroidal melanoma

We aimed to describe the feasibility and efficacy of a novel non-invasive fixation and monitoring (F-M) device for the eyeballs (which uses a right-angle prism mirror as the optic axis guide) in three consecutive patients with choroidal melanoma who were treated with intensity-modulated radiotherapy (IMRT). The device consists of an immobilization shell, a right-angle prism mirror, a high magnification optical zoom video camera, a guide lamp, a digital voice recorder, a personal computer, and a National Television System Committee standard analog video cable. Using the right-angle prism mirror, the antero-posterior axis was determined coincident with the optic axis connecting the centers of the cornea and pupil. The axis was then connected to the guide light and video camera installed on the couch top on the distal side. Repositioning accuracy improved using this method. Furthermore, the positional error of the lens was markedly reduced from ±1.16, ±1.68 and ±1.11 mm to ±0.23, ±0.58 and ±0.26 mm in the horizontal direction, and from ±1.50, ±1.03 and ±0.48 mm to ±0.29, ±0.30 and ±0.24 mm in the vertical direction (Patient #1, #2 and #3, respectively). Accordingly, the F-M device method decreased the planning target volume size and improved the dose-volume histogram parameters of the organ-at-risk via IMRT inverse planning. Importantly, the treatment method was well tolerated.

Iridium-Knife: Another knife in radiation oncology

PURPOSE

Intratarget dose escalation with superior conformity is a defining feature of three-dimensional (3D) iridium-192 (¹⁹²Ir) high-dose-rate (HDR) brachytherapy (BRT). In this study, we analyzed the dosimetric characteristics of interstitial ¹⁹²Ir HDR BRT for intrathoracic and cerebral malignancies. We examined the dose gradient sharpness of HDR BRT compared with that of linear accelerator-based stereotactic radiosurgery and stereotactic body radiation therapy, usually called X-Knife, to demonstrate that it may as well be called a Knife.

METHODS AND MATERIALS

Treatment plans for 10 patients with recurrent glioblastoma multiforme or intrathoracic malignancies, five of each entity, treated with X-Knife (stereotactic radiosurgery for glioblastoma multiforme and stereotactic body radiation therapy for intrathoracic malignancies) were replanned for simulated HDR BRT. For 3D BRT planning, we used identical structure sets and dose prescription as for the X-Knife planning. The indices for qualitative treatment plan analysis encompassed planning target volume coverage, conformity, dose falloff gradient, and the maximum dose-volume limits to different organs at risk.

RESULTS

Volume coverage in HDR plans was comparable to that calculated for X-Knife plans with no statistically significant difference in terms of conformity. The dose falloff gradient-sharpness-of the HDR plans was considerably steeper compared with the X-Knife plans.

CONCLUSIONS

Both 3D ¹⁹²Ir HDR BRT and X-Knife are effective means for intratarget dose escalation with HDR BRT achieving at least equal conformity and a steeper dose falloff at the target volume margin. In this sense, it can reasonably be argued that 3D ¹⁹²Ir HDR BRT deserves also to be called a Knife, namely Iridium-Knife.

Stereotactic Radiosurgery and Fractionated Stereotactic Radiotherapy in the Treatment of Uveal Melanoma

Uveal melanoma is the most common primary intraocular malignant tumor. Radiation therapy has now replaced enucleation as the treatment of choice, with radioactive eye plaques and proton therapy being the two most studied radiotherapy modalities. More recently, stereotactic radiosurgery and fractionated stereotactic radiotherapy have emerged as promising, non-invasive treatments for uveal melanoma. This review summarizes the available literature on these newer treatment modalities.

Dose-response of critical structures in the posterior eye segment to hypofractioned stereotactic photon radiotherapy of choroidal melanoma

PURPOSE

To identify modifying factors and dose-/volume-response relationships for the retina and optic nerve related to highly conformal hypofractionated radiotherapy.

PATIENTS AND METHODS

Seventy-three patients undergoing hypofractionated stereotactic photon radiotherapy of choroidal melanoma were included in this retrospective study. The volumes of the optic nerve receiving doses of more than 7.5 or 12 Gy, respectively, were defined. Optic nerve circumference included in the 30%, 40%, 50%, or 80% isodose (ON%) and retina included in 30% or 40% were determined as quantal effects. Univariate and multivariate analyses were performed for clinical variables as well as probit analysis to define EDx (doses where a positive response is expected in x% of the cases).

RESULTS

Median follow-up was 90.0 (interquartile range 69.0-98.0) months. Fifty-two (71%) and 49 (67%) patients developed radiation retinopathy and optic neuropathy (any grade). Age, length of follow-up and diabetes were significant parameters regarding retinopathy. Optic neuropathy was significantly influenced by age, length of follow-up, and ON30. The probability of optic neuropathy (any grade and grade ≥ 2) significantly increases with the dose (p ranges from 0.0126 to 0.0211).

CONCLUSION

Treatment planning should aim at minimizing encompassing isodoses particularly in the low dose region, without compromising PTV coverage.

[Choroidal melanoma treated by protontherapy: anatomical and functional results at Strasbourg University Medical Center]

PURPOSE

To report anatomical and functional results of choroidal melanoma treated by protontherapy between 2001 and 2011 and to compare these with data reported in the literature.

METHODS

The present study is a retrospective review of the charts of patients presenting with choroidal melanoma, managed at Strasbourg University Medical Center between 2001 and 2011, and receiving protontherapy. Visual acuity and a complete ophthalmologic examination, including intraocular pressure, dilated fundus exam, and ultrasonography with measurement of the major diameter of the tumor, were performed prior to treatment and at various follow-up visits (every six months for five years, then once a year for ten years). The rates and timing of local complications (cataract, rubeosis, retinal detachment, neovascular glaucoma and secondary enucleation) and systemic complications (metastasis) are also reported.

RESULTS

Seventy charts were included. Mean age of the treated population was 61.4 ± 15.2 years. Male/female ratio was 1.06. Mean visual acuity, 0.55 ± 0.6 (20/67) on presentation, decreased progressively from month 42. Mean tumor thickness was 5.8 ± 2.8mm and major diameter 11.2 ± 2.9 mm. Decreased tumor thickness was noted beginning at month 12 of follow-up (Anova, P<0.0001), while major diameter remained unchanged over time (P>0.05). The risk of complications (retinal detachment, rubeosis, neovascular glaucoma) was greater between the second and third year. Mean 5-year survival was 87.5%. The rate of secondary enucleation was 8.5%.

CONCLUSION

Protontherapy most often allows for preservation of the globe. Visual loss, often significant and permanent, is frequent.

Current status and future perspective of flattening filter free photon beams

PURPOSE

Flattening filters (FFs) have been considered as an integral part of the treatment head of a medical accelerator for more than 50 years. The reasons for the longstanding use are, however, historical ones. Advanced treatment techniques, such as stereotactic radiotherapy or intensity modulated radiotherapy have stimulated the interest in operating linear accelerators in a flattening filter free (FFF) mode. The current manuscript reviews treatment head physics of FFF beams, describes their characteristics and the resulting potential advantages in their medical use, and closes with an outlook.

METHODS

A number of dosimetric benefits have been determined for FFF beams, which range from increased dose rate and dose per pulse to favorable output ratio in-air variation with field size, reduced energy variation across the beam, and reduced leakage and out-of-field dose, respectively. Finally, the softer photon spectrum of unflattened beams has implications on imaging strategies and radiation protection.

RESULTS

The dosimetric characteristics of FFF beams have an effect on treatment delivery, patient comfort, dose calculation accuracy, beam matching, absorbed dose determination, treatment planning, machine specific quality assurance, imaging, and radiation protection. When considering conventional C-arm linacs in a FFF mode, more studies are needed to specify and quantify the clinical advantages, especially with respect to treatment plan quality and quality assurance.

CONCLUSIONS

New treatment units are already on the market that operate without a FF or can be operated in a dedicated clinical FFF mode. Due to the convincing arguments of removing the FF, it is expected that more vendors will offer dedicated treatment units for advanced photon beam therapy in the near future. Several aspects related to standardization, dosimetry, treatment planning, and optimization need to be addressed in more detail in order to facilitate the clinical implementation of unflattened beams.

Local tumor control, visual acuity, and survival after hypofractionated stereotactic photon radiotherapy of choroidal melanoma in 212 patients treated between 1997 and 2007

PURPOSE

To evaluate long-term local tumor control, visual acuity, and survival after hypofractionated linear accelerator-based stereotactic photon radiotherapy in patients with choroidal melanoma.

METHODS AND MATERIALS

Between 1997 and 2007, 212 patients with choroidal melanoma unsuitable for ruthenium-106 brachytherapy or local resection were treated stereotactically at a linear accelerator with 6-MV photon beams at the Medical University of Vienna in five fractions over 7 days. Twenty-four patients received a total dose of 70 Gy (five fractions of 14 Gy), 158 a total dose of 60 Gy (five fractions of 12 Gy) and 30 patients a total dose of 50 Gy (five fractions of 10 Gy) applied on the 80% isodose. Ophthalmologic examinations were performed at baseline and every 3 months in the first 2 years, every 6 months until 5 years, and once a year thereafter until 10 years after radiotherapy. Assessment of visual acuity, routine ophthalmologic examinations, and measurement of tumor base dimension and height using standardized A-scan and B-scan echography were done at each visit. Funduscopy and fluorescein angiography were done when necessary to document tumor response.

RESULTS

Median tumor height and volume decreased from 4.8 mm and 270.7 mm3 at baseline to 2.6 mm and 86.6 mm3 at the last individual follow-up, respectively (p<0.001, p<0.001). Median visual acuity decreased from 0.55 at baseline to hand motion at the last individual follow-up (p<0.001). Local tumor control was 95.9% after 5 years and 92.6% after 10 years. Thirty-two patients developed metastatic disease, and 22 of these patients died during the follow-up period.

CONCLUSION

Hypofractionated stereotactic photon radiotherapy with 70 to 50 Gy delivered in five fractions in 7 days is sufficient to achieve excellent local tumor control in patients with malignant melanoma of the choroid. Disease outcome and vision are comparable to those achieved with proton beam radiotherapy. Decreasing the total dose below 60 Gy seems to be possible.

Dosimetric performance of the new high-definition multileaf collimator for intracranial stereotactic radiosurgery

The objective was to evaluate the performance of a high-definition multileaf collimator (MLC) of 2.5 mm leaf width (MLC2.5) and compare to standard 5 mm leaf width MLC (MLC5) for the treatment of intracranial lesions using dynamic conformal arcs (DCA) technique with a dedicated radiosurgery linear accelerator. Simulated cases of spherical targets were created to study solely the effect of target volume size on the performance of the two MLC systems independent of target shape complexity. In addition, 43 patients previously treated for intracranial lesions in our institution were retrospectively planned using DCA technique with MLC2.5 and MLC5 systems. The gross tumor volume ranged from 0.07 to 40.57 cm3 with an average volume of 5.9 cm3. All treatment parameters were kept the same for both MLC-based plans. The plan evaluation was performed using figures of merits (FOM) for a rapid and objective assessment on the quality of the two treatment plans for MLC2.5 and MLC5. The prescription isodose surface was selected as the greatest isodose surface covering >or= 95% of the target volume and delivering 95% of the prescription dose to 99% of target volume. A Conformity Index (CI) and conformity distance index (CDI) were used to quantifying the dose conformity to a target volume. To assess normal tissue sparing, a normal tissue difference (NTD) was defined as the difference between the volume of normal tissue receiving a certain dose utilizing MLC5 and the volume receiving the same dose using MLC2.5. The CI and normal tissue sparing for the simulated spherical targets were better with the MLC2.5 as compared to MLC5. For the clinical patients, the CI and CDI results indicated that the MLC2.5 provides better treatment conformity than MLC5 even at large target volumes. The CI's range was 1.15 to 2.44 with a median of 1.59 for MLC2.5 compared to 1.60-2.85 with a median of 1.71 for MLC5. Improved normal tissue sparing was also observed for MLC2.5 over MLC5, with the NTD always positive, indicating improvement, and ranging from 0.1 to 8.3 for normal tissue receiving 50% (NTV50), 70% (NTV70) and 90% (NTV90) of the prescription dose. The MLC2.5 has a dosimetric advantage over the MLC5 in Linac-based radiosurgery using DCA method for intracranial lesions, both in treatment conformity and normal tissue sparing when target shape complexity increases.

Dosimetric comparison of intensity-modulated stereotactic radiotherapy with other stereotactic techniques for locally recurrent nasopharyngeal carcinoma

PURPOSE

Locally recurrent nasopharyngeal carcinoma (NPC) patients can be salvaged by reirradiation with a substantial degree of radiation-related complications. Stereotactic radiotherapy (SRT) is widely used in this regard because of its rapid dose falloff and high geometric precision. The aim of this study was to examine whether the newly developed intensity-modulated stereotactic radiotherapy (IMSRT) has any dosimetric advantages over three other stereotactic techniques, including circular arc (CARC), static conformal beam (SmMLC), and dynamic conformal arc (mARC), in treating locally recurrent NPC.

METHODS AND MATERIALS

Computed tomography images of 32 patients with locally recurrent NPC, previously treated with SRT, were retrieved from the stereotactic planning system for contouring and computing treatment plans. Treatment planning of each patient was performed for the four treatment techniques: CARC, SmMLC, mARC, and IMSRT. The conformity index (CI) and homogeneity index (HI) of the planning target volume (PTV) and doses to the organs at risk (OARs) and normal tissue were compared.

RESULTS

All four techniques delivered adequate doses to the PTV. IMSRT, SmMLC, and mARC delivered reasonably conformal and homogenous dose to the PTV (CI <1.47, HI <0.53), but not for CARC (p < 0.05). IMSRT presented with the smallest CI (1.37) and HI (0.40). Among the four techniques, IMSRT spared the greatest number of OARs, namely brainstem, temporal lobes, optic chiasm, and optic nerve, and had the smallest normal tissue volume in the low-dose region.

CONCLUSION

Based on the dosimetric comparison, IMSRT was optimal for locally recurrent NPC by delivering a conformal and homogenous dose to the PTV while sparing OARs.

Dosimetric comparison of helical tomotherapy and dynamic conformal arc therapy in stereotactic radiosurgery for vestibular schwannomas

The dosimetric results of stereotactic radiosurgery (SRS) for vestibular schwannoma (VS) performed using dynamic conformal arc therapy (DCAT) with the Novalis system and helical TomoTherapy (HT) were compared using plan quality indices. The HT plans were created for 10 consecutive patients with VS previously treated with SRS using the Novalis system. The dosimetric indices used to compare the techniques included the conformity index (CI) and homogeneity index (HI) for the planned target volume (PTV), the comprehensive quality index (CQI) for nine organs at risk (OARs), gradient score index (GSI) for the dose drop-off outside the PTV, and plan quality index (PQI), which was verified using the plan quality discerning power (PQDP) to incorporate 3 plan indices, to evaluate the rival plans. The PTV ranged from 0.27-19.99 cm(3) (median 3.39 cm(3)), with minimum required PTV prescribed doses of 10-16 Gy (median 12 Gy). Both systems satisfied the minimum required PTV prescription doses. HT conformed better to the PTV (CI: 1.51 ± 0.23 vs. 1.94 ± 0.34; p < 0.01), but had a worse drop-off outside the PTV (GSI: 40.3 ± 10.9 vs. 64.9 ± 13.6; p < 0.01) compared with DCAT. No significant difference in PTV homogeneity was observed (HI: 1.08 ± 0.03 vs. 1.09 ± 0.02; p = 0.20). HT had a significantly lower maximum dose in 4 OARs and significant lower mean dose in 1 OAR; by contrast, DCAT had a significantly lower maximum dose in 1 OAR and significant lower mean dose in 2 OARs, with the CQI of the 9 OARs = 0.92 ± 0.45. Plan analysis using PQI (HT 0.37 ± 0.12 vs. DCAT 0.65 ± 0.08; p < 0.01), and verified using the PQDP, confirmed the dosimetric advantage of HT. However, the HT system had a longer beam-on time (33.2 ± 7.4 vs. 4.6 ± 0.9 min; p < 0.01) and consumed more monitor units (16772 ± 3803 vs. 1776 ± 356.3; p < 0.01). HT had a better dose conformity and similar dose homogeneity but worse dose gradient than DCAT. Plan analysis confirmed the dosimetric advantage of HT, although not all indices revealed a better outcome for HT. Whether this dosimetric advantage translates into a clinical benefit deserves further investigation.

Impact of collimator leaf width and treatment technique on stereotactic radiosurgery and radiotherapy plans for intra- and extracranial lesions

Background

This study evaluated the dosimetric impact of various treatment techniques as well as collimator leaf width (2.5 vs 5 mm) for three groups of tumors – spine tumors, brain tumors abutting the brainstem, and liver tumors. These lesions often present challenges in maximizing dose to target volumes without exceeding critical organ tolerance. Specifically, this study evaluated the dosimetric benefits of various techniques and collimator leaf sizes as a function of lesion size and shape.

Methods

Fifteen cases (5 for each site) were studied retrospectively. All lesions either abutted or were an integral part of critical structures (brainstem, liver or spinal cord). For brain and liver lesions, treatment plans using a 3D-conformal static technique (3D), dynamic conformal arcs (DARC) or intensity modulation (IMRT) were designed with a conventional linear accelerator with standard 5 mm leaf width multi-leaf collimator, and a linear accelerator dedicated for radiosurgery and hypofractionated therapy with a 2.5 mm leaf width collimator. For the concave spine lesions, intensity modulation was required to provide adequate conformality; hence, only IMRT plans were evaluated using either the standard or small leaf-width collimators.

A total of 70 treatment plans were generated and each plan was individually optimized according to the technique employed. The Generalized Estimating Equation (GEE) was used to separate the impact of treatment technique from the MLC system on plan outcome, and t-tests were performed to evaluate statistical differences in target coverage and organ sparing between plans.

Results

The lesions ranged in size from 2.6 to 12.5 cc, 17.5 to 153 cc, and 20.9 to 87.7 cc for the brain, liver, and spine groups, respectively. As a group, brain lesions were smaller than spine and liver lesions. While brain and liver lesions were primarily ellipsoidal, spine lesions were more complex in shape, as they were all concave. Therefore, the brain and the liver groups were compared for volume effect, and the liver and spine groups were compared for shape. For the brain and liver groups, both the radiosurgery MLC and the IMRT technique contributed to the dose sparing of organs-at-risk(OARs), as dose in the high-dose regions of these OARs was reduced up to 15%, compared to the non-IMRT techniques employing a 5 mm leaf-width collimator. Also, the dose reduction contributed by the fine leaf-width MLC decreased, as dose savings at all levels diminished from 4 – 11% for the brain group to 1 – 5% for the liver group, as the target structures decreased in volume. The fine leaf-width collimator significantly improved spinal cord sparing, with dose reductions of 14 – 19% in high to middle dose regions, compared to the 5 mm leaf width collimator.

Conclusion

The fine leaf-width MLC in combination with the IMRT technique can yield dosimetric benefits in radiosurgery and hypofractionated radiotherapy. Treatment of small lesions in cases involving complex target/OAR geometry will especially benefit from use of a fine leaf-width MLC and the use of IMRT.

Dosimetric characteristics of dual-layer multileaf collimation for small-field and intensity-modulated radiation therapy applications

The purpose of the present work was to measure the performance characteristics in the penumbra region and on the leaf‐end of an innovative dual‐layer micro multileaf collimator (DmMLC). The DmMLC consists of two orthogonal (upper and lower) layers of leaves; a standard MLC consists of one layer. The DmMLC provides unique performance characteristics in smoothing dose undulation, reducing leaf‐end transmission, and reducing MLC field dependence of the leaf stepping angle. Two standard MLCs (80‐leaf and 120‐leaf versions: Varian Medical Systems, Palo Alto, CA), a DmMLC (AccuKnife: Initia Medical Technology, Canton, MA), and a Cerrobend (Cerro Metal Products, Bellefonte, PA) block were used in performance studies involving a triangular field, a cross leaf‐end field, and a circular field. Measurements were made with 6‐MV X‐rays and extended dose range film at a depth of 5 cm in Solid Water (Gammex rmi, Middleton, WI) at a source–axis distance of 100 cm. The field penumbra width measured between the 20% and 80% isodose lines through the MLC‐80, MLC‐120, DmMLC, and Cerrobend block were 9.0, 5.0, 3.0, and 2.0 mm respectively. The dose undulation amplitude of the 50% isodose line was measured as 5.5, 2.0, and 0.5 mm for the MLC‐80, MLC‐120, and DmMLC respectively. The planar dose difference between the MLC‐80, MLC‐120, and DmMLC against Cerrobend block was measured as ranging at ±52.5%,±35.0%, and ±20.0% respectively. The leaf‐end transmission was measured at 22.4% in maximum and 15.4% in average when closing a single layer of the DmMLC, and at 2.4% in maximum and 2.1% in average when closing both layers. The MLC dependence of the leaf stepping angle with the DmMLC ranged from 45 degrees to 90 degrees. The standard MLC leaf stepping angle ranged from 0 degrees to 90 degrees. In conclusion, the dose undulation, leaf‐end transmission, and MLC field dependence of the leaf stepping angle with the DmMLC were remarkably reduced as compared with those of the standard MLCs. And as compared with Cerrobend block, the DmMLC provided very comparable performance in field‐edge smoothing and in the shaping of complex fields.

PACS numbers: 87.56.Jk, 87.56.Nk, 87.56.Nj, 87.57.Nt

Hearing preservation following fractionated stereotactic radiotherapy for vestibular schwannomas: prognostic implications of cochlear dose

Object

The goal in this study was to evaluate hearing preservation rates and to determine prognostic factors for this outcome following fractionated stereotactic radiotherapy (FSRT) of vestibular schwannoma.

Methods

Thirty-four consecutive patients with serviceable hearing who received FSRT between May 1998 and December 2003 were identified. Clinical and audiometry data were collected prospectively. The prescription dose was 45 Gy in 25 fractions prescribed to the 90% isodose line. The median follow-up duration was 36.5 months (range 12–85 months). The actuarial 2- and 4-year local control rates were 100 and 95.7%, respectively. Permanent trigeminal and facial nerve complications were 0 and 6%, respectively. The actuarial 2- and 3-year serviceable hearing preservation rates were both 63%. The median loss in speech reception threshold was 15 dB (range −10 to 65 dB). The radiotherapy dose to the cochlea was the only significant prognostic factor for hearing deterioration. Radiotherapy dose to the cochlear nucleus, patient age, sex, pre-FSRT hearing grade, tumor volume, and intracanalicular tumor volume failed to show any significance as prognostic factors.

Results

Five cases were replanned with four different radiotherapy techniques (namely arcs, dynamic arcs, static conformal fields, and intensity-modulated radiotherapy), with the cochlea defined as an organ at risk. In all cases, replanning resulted in statistically significant reduction in radiation to the cochlea (p = 0.001); however, no single replanning technique was found to be superior.

Conclusions

The radiation dose to the cochlea is strongly predictive for subsequent hearing deterioration. It is essential for the cochlea to be outlined as an organ at risk, and for radiation techniques to be optimized, to improve long-term hearing preservation.

Stereotactic radiotherapy for ocular melanoma: initial experience using closed eyes for ocular target immobilization

To assess the reliability and target positioning reproducibility with eyes closed in uveal melanoma patients treated with a micromultileaf-based linear accelerator dedicated for stereotactic radiotherapy. Five consecutive patients treated with curative radiotherapy for uveal melanoma were monitored for positioning reproducibility with resimulation CT scans performed every two days while on treatment (23 resimulation CTs available). All patients underwent MRIs of the orbits before simulation to help to define the target and organs at risk (e.g., lenses, optic nerves, ciliary bodies, and lacrimal glands) in the simulation CT (MRI-to-CT bone registration). Patients were simulated, resimulated, and treated with eyes closed. Patient #1 was treated with 5 daily fractions while patients #2 to #5, were treated with 10 daily fractions. We chose the lens of the tumor-bearing eye as the structure to be controlled, assuming that correct repositioning of the lens should be a valid surrogate for correctness of target repositioning. Displacements (mean and standard deviations, SD) of the lens in the three axes were measured for each patient. Systematic and standard errors were calculated. Planning target volume (PTV) margins were estimated according to McKenzie et al. [Phys Med Biol 45, 3331-3342 (2000)]. For both AP-PA and left-right shifts calculated SD were always below 1 mm, except for patient #4, who was treated with a non-customized bolus that pushed the globe backwards in a random fashion. In ideal set-up conditions PTV margins around the target were estimated to be 3 mm. Asking patients to close their eyes is a simple and reliable immobilization procedure when treating ocular tumors with stereotactic radiotherapy. Margins of 3 mm around the target may be necessary to safely treat these tumors under ideal set-up conditions.

Does Intensity Modulation Improve Healthy Tissue Sparing in Stereotactic Radiosurgery of Complex Arteriovenous Malformations?

This planning study evaluates the potential of intensity modulated treatment fields and inverse planning techniques in stereotactic radiosurgery to reduce healthy tissue dose. Twenty patients previously treated with stereotactic radiosurgery for arteriovenous malformation (AVM) were replanned with each of 4 techniques: circular non-coplanar arcs, dynamic arcs, static conformal fields, and intensity modulated radiosurgery (IMRS). Patients were selected having a maximum AVM dimension at least 20 mm, or volume greater than 10 cm(3). Target volumes ranged from 2.12 cm(3) to 13.87 cm(3) with a median of 6.03 cm(3). Resulting dose distributions show a statistically significant improvement in target conformality between circular arcs and all other techniques (p </= 0.001), between conformal and both dynamic arcs and IMRS (p </= 0.03) and with no difference between dynamic arcs and IMRS. However, for AVMs of volume greater than 5.5 cm(3), IMRS gives better conformality than dynamic arcs (p = 0.04). IMRS showed consistently lower dose inhomogeneity compared to both dynamic arcs and conformal fields (p < 0.001). At low dose levels, the dynamic arc technique irradiates less healthy tissue than the other techniques (p </= 0.001). Both dynamic arcs and IMRS provide increased ability to conform to the AVM, with IMRS showing greater ability to control dose at the periphery.

Proton beam radiotherapy versus fractionated stereotactic radiotherapy for uveal melanomas: A comparative study

PURPOSE

A comparative treatment planning study was undertaken between proton and photon therapy in uveal melanoma to assess the potential benefits and limitations of these treatment modalities. A fixed proton horizontal beam (OPTIS) and intensity-modulated spot-scanning proton therapy (IMPT), with multiple noncoplanar beam arrangements, was compared with linear accelerator-based stereotactic radiotherapy (SRT), using a static and a dynamic micromultileaf collimator and intensity-modulated RT (IMRS).

METHOD AND MATERIALS

A planning CT scan was performed on a brain metastasis patient, with a 3-mm acquisition slice spacing and the patient looking at a luminous spot with the eyes in three different positions (neutral and 25 degrees right and left). Four different gross tumor volumes were defined for each treatment technique. These target scenarios represented different locations (involving vs. not involving the macula and temporal vs. nasal) and volumes (10 x 6 mm vs. 16 x 10 mm) to challenge the proton and photon treatment techniques. The planning target volume was defined as the gross tumor volume plus 2 mm laterally and 3 mm craniocaudally for both modalities. A dose homogeneity of 95-99% of the planning target volume was used as the "goal" for all techniques. The dose constraint (maximum) for the organs at risk (OARs) for both the proton and the SRT photon plans was 27.5, 22.5, 20, and 9 CGE-Gy for the optic apparatus, retina, lacrimal gland, and lens, respectively. The dose to the planning target volume was 50 CGE-Gy in 10 CGE-Gy daily fractions. The plans for proton and photon therapy were computed using the Paul Scherrer Institute and BrainSCAN, version 5.2 (BrainLAB, Heimstetten, Germany) treatment planning systems, respectively. Tumor and OARs dose-volume histograms were calculated. The results were analyzed using the dose-volume histogram parameters, conformity index (CI(95%)), and inhomogeneity coefficient.

RESULTS

Target coverage of all simulated uveal melanomas was equally conformal with the photon and proton modalities. The median CI(95%) value was 1.74, 1.86, and 1.83 for the static, dynamic, and IMSRT plans, respectively. With proton planning, the median CI(95%) was 1.88 for OPTIS and substantially improved with IMPT in some tumor cases (median CI(95%), 1.29). The tumor dose homogeneity in the proton plans was, however, always better than with SRT photon planning (median inhomogeneity coefficient 0.1 and 0.15 vs. 0.46, 0.41, and 0.23 for the OPTIS and IMPT vs. the static, dynamic, and IMSRT plans, respectively). Compared with the photon plans, the use of protons did not lead to a substantial reduction in the homolateral OAR total integral dose in the low- to high-dose level, except for the lacrimal gland. The median maximal dose and dose at the 10% volume with the static, dynamic, and IMSRT plans was 33-30.8, 31.8-28, and 35.8-49 Gy, respectively, for the lacrimal gland, a critical organ. For protons, only the OPTIS plans were better, with a median maximal dose and dose at the 10% volume using OPTIS and IMPT of 19.2 and 8.8 and 25.6 and 23.6 CGE, respectively. The contralateral OARs were completely spared with the proton plans, but the median dose delivered to these structures was 1.2 Gy (range, 0-6.3 Gy) with the SRT photon plans.

CONCLUSION

These results suggest that the use of SRT photon techniques, compared with protons, can result in similar levels of dose conformation. IMPT did not increase the degree of conformality for this small tumor. Tumor dose inhomogeneity was, however, always increased with photon planning. Except for the lacrimal gland, the use of protons, with or without intensity modulation, did not increase homolateral OAR dose sparing. The dose to all the contralateral OARs was, however, completely eliminated with proton planning.

Impact of IMRT and leaf width on stereotactic body radiotherapy of liver and lung lesions

PURPOSE

The present study explored the impact of intensity-modulated radiotherapy (IMRT) on stereotactic body RT (SBRT) of liver and lung lesions. Additionally, because target dose conformity can be affected by the leaf width of a multileaf collimator (MLC), especially for small targets and stereotactic applications, the use of a micro-MLC on "uniform intensity" conformal and intensity-modulated SBRT was evaluated.

METHODS AND MATERIALS

The present study included 10 patients treated previously with SBRT in our institution (seven lung and three liver lesions). All patients were treated with 3 x 12 Gy prescribed to the 65% isodose level. The actual MLC-based conformal treatment plan served as the standard for additional comparison. In total, seven alternative treatment plans were made for each patient: a standard (actual) plan and an IMRT plan, both calculated with Helax TMS (Nucletron) using a pencil beam model; and a recalculated standard and a recalculated IMRT plan on Helax TMS using a point dose kernel approach. These four treatment plans were based on a standard MLC with 1-cm leaf width. Additionally, the following micro-MLC (central leaf width 3 mm)-based treatment plans were calculated with the BrainSCAN (BrainLAB) system: standard, IMRT, and dynamic arc treatments. For each treatment plan, various target parameters (conformity, coverage, mean, maximal, and minimal target dose, equivalent uniform doses, and dose-volume histogram), as well as organs at risk parameters (3 Gy and 6 Gy volume, mean dose, dose-volume histogram) were evaluated. Finally, treatment efficiency was estimated from monitor units and the number of segments for IMRT solutions.

RESULTS

For both treatment planning systems, no significant difference could be observed in terms of target conformity between the standard and IMRT dose distributions. All dose distributions obtained with the micro-MLC showed significantly better conformity values compared with the standard and IMRT plans using a regular MLC. Dynamic arc plans were characterized by the steepest dose gradient and thus the smallest V(6 Gy) values, which were on average 7% smaller than the standard plans and 20% lower than the IMRT plans. Although the Helax TMS IMRT plans show about 18% more monitor units than the standard plan, BrainSCAN IMRT plans require approximately twice the number of monitor units relative to the standard plan. All treatment plans optimized with a pencil beam model but recalculated with a superposition method showed significant qualitative, as well as quantitative, differences, especially with respect to conformity and the dose to organs at risk.

CONCLUSION

Standard conformal treatment techniques for SBRT could not be improved with inversely planned IMRT approaches. Dose calculation algorithms applied in optimization modules for IMRT applications in the thoracic region need to be based on the most accurate dose calculation algorithms, especially when using higher energy photon beams.

Dosimetric study using different leaf-width MLCs for treatment planning of dynamic conformal arcs and intensity-modulated radiosurgery

This paper systematically studied the dosimetric difference between a 3 mm micro multileaf collimator (MLC), a 5 mm MLC, and a 10 mm MLC for stereotactic radiosurgery using the Brainscan treatment planning system. Thirty-four cases treated with the dynamic conformal arcs technique and 20 cases treated with the intensity modulated radiosurgery/fractionated radiotherapy (IMRS/ IMRT) technique were retrospectively studied. The conformity index, the percentage target coverage, and the dose-volume histogram (DVH) for organs-at-risk (OARs) were used for dosimetric analysis and comparison for different treatment techniques, target volumes, and treatment sites. For the dynamic conformal arcs technique, there were statistically significant differences in the conformity indices between different leaf-width MLCs. The ratio of the conformity indices between different MLCs depended on the target volume. The average conformity index ratios between the 5 mm MLC and the 3 mm MLC were 1.37+/-0.09, 1.12+/-0.04, 1.08+/-0.02 and 1.04+/-0.01, respectively, for patients with the target volume (V) in groups: (1) V< 1 cm3, (2) 1 cm3 < V< 8 cm3, (3) 8 cm3 < V< 27 cm3, and (4) V> 27 CC. The average conformity index ratios between the 10 and 3 mm MLCs were 2.00+/-0.33, 1.45+/-0.09, 1.28+/-0.09, and 1.18+/-0.05 for patients in these four volume groups, respectively. No statistically significant difference was found for the target coverage among different MLCs. For the IMRS/IMRT technique, the average conformity index and target coverage ratios were 1.01+/-0.05 and 1.00+/-0.02, respectively, between the 5 and 3 mm MLCs, and were 1.04+/-0.07 and 0.97+/-0.02, respectively, between the 10 and 3 mm MLCs. The 3 mm MLC showed slightly better overall OAR DVHs than the 5 and 10 mm MLCs, especially for the cranial site with small-volume OARs defined. The results suggest that for the dynamic conformal arcs technique, the narrower leaf-width MLC provides better dose conformity than the wider leaf-width MLCs. This advantage decreases when the target volume increases. For the IMRS/IMRT technique, the narrower leaf-width MLC could have better sparing of small OARs than the wider leaf-width MLC.

Stereotactic arc therapy for small elongated tumors using cones and collimator jaws; dosimetric and planning aspects

Stereotactic arc treatment of small intracranial tumors is usually performed with arcs collimated by circular cones, resulting in treatment volumes which are basically spherical. For nonspherical lesions this results in a suboptimal dose distribution. Multiple isocenters may improve the dose conformity for these lesions, at the cost of large overdosages in the target volume. To achieve improved dose conformity as well as dose homogeneity, the linac jaws (with a minimum distance of 1.0 cm to the central beam axis) can routinely be used to block part of the circular beams. The purpose of this study was to investigate the feasibility of blocking cones with diameters as small as 1.0 cm and a minimum distance between the jaw and the central beam axis of 0.3 cm. First, the reproducibility in jaw positioning and resulting dose delivery on the treatment unit were assessed. Second, the accuracy of the TPS dose calculation for these small fields was established. Finally, clinically applied treatment plans using nonblocked cones were compared with plans using the partially blocked cones for several treatment sites. The reproducibility in dose delivery on our Varian Clinac 2300 C/D machines on the central beam axis is 0.8% (1 SD). The accuracy of the treatment planning system dose calculation algorithm is critically dependent on the used fits for the penumbra and the phantom scatter. The average deviation of calculated from measured dose on the central beam axis is -1.0%+/-1.4% (1 SD), which is clinically acceptable. Partial cone blocking results in improved dose distributions for elongated tumors, such as vestibular schwannoma and uveal melanoma. Multiple isocenters may be avoided. The technique is easy to implement and requires no additional workload.

Automatic real-time surveillance of eye position and gating for stereotactic radiotherapy of uveal melanoma

A new prototype (hardware and software) for monitoring eye movements using a noninvasive technique for gated linac-based stereotactic radiotherapy (SRT) of uveal melanoma was developed. The prototype was tested within the scope of a study for 11 patients. Eye immobilization was achieved by having the patient fixate a light source integrated into the system. The system is used in conjunction with a Head&Neck mask system for immobilization, and uses infrared tracking technology for positioning (both BrainLAB AG Heimstetten/Germany). It was used during CT and MR image acquisition as well as during all of five treatment fractions (6 MeV, 5 x 12 Gy to 80% isodose) to guarantee identical patient setup and eye rotational state during treatment planning and treatment delivery. Maximum temporal and angular deviations tolerated during treatment delivery can be chosen by the physician, the radiation then being interrupted automatically and instantaneously if those criteria are being exceeded during irradiation. A graphical user interface displays life video images of the treated eye and information about the current and previous rotational deviation of the eye from its reference treatment position. The physician thus has online access to data directly linked to the success of the treatment and possible side effects. Mean angular deviations during CT/MR scans and treatment deliveries ranged from 1.61 degrees to 3.64 degrees (standard deviations 0.87 degrees to 2.09 degrees ) which is in accordance with precision requirements for SRT. Typical situations when preset deviation criteria were exceeded are slow drifts (fatigue), sudden large eye movements (irritation), or if patients closed their eyes (fatigue). In these cases radiation was reliably interrupted by the gating system. In our clinical setup the novel system for computer-controlled eye movement gated treatments was well tolerated by all patients. The system yields quantitative real-time information about the eye's rotational state with respect to a reference position (treatment planning situation). Together with the possibility of performing movement-gated treatments of uveal melanoma, the system thus greatly improves the quality of this treatment.

A new approach in dose measurement and error analysis for narrow photon beams (beamlets) shaped by different multileaf collimators using a small detector

Dose measurement for narrow stereotactic beams and intensity modulation radiotherapy beamlets is difficult and error-prone due to the lack of lateral electron equilibrium. A small detector position error and finite sensitive volume as well as the nonfocus collimation could result in considerable (> 10%) measurement errors. A new method is introduced here to measure the dose and error components so that the accuracy and precision of the dose measurement can be improved. Based on superposition principle, we can create exactly the small field of interest by subtraction of a reference open field (O-field) and two strip fields (S-fields) from the sum of four quadrant fields (Q-fields). The position effect on the dose measurement is determined by the standard deviation of the four Q-field readings. The collimator leaf-edge effect (LEE) is quantified by the difference between the readings of the two S-fields using a detector that has very small sensitive volume. The detector-volume effect can be analytically estimated from the integrals of the dose distributions of the two S-fields over the detector volume. Using a pinpoint ion chamber (PTW N31006) and a stereotactic silicon diode detector (Scanditronix, DEB050), we have measured scatter factors (SF) and tissue-maximum ratios for 6-MV x-ray fields with sizes of 3 x 3 and 6 x 6 mm2 shaped by a BrainLAB micromultileaf collimator (microMLC) (M3), 4.4 x 4.4 and 8.8 x 8.8 mm2 shaped by a 3DLine double-focus MLC, and 5 x 5 and 10 x 10 mm2 by a Varian Millennium MLC. Our experimental results demonstrate that the large errors are often caused by a small setup error or measuring point displacement from the central ray of the beam. The LEE is almost independent of the depth but closely related to the field size and the type of MLC. The volume effect becomes significant when the detector diameter is comparable to the half size of the small fields. Application of the new method using different detectors had achieved less than 8.3% total experiment error for all of the small fields of interest except for the SF of the 3 x 3 mm2 field from the pinpoint ion chamber that has 15% volume effect. Importantly, the new method using a solid water phantom is clinically convenient and highly reproducible.